As part of applications such as micro-electronic and micro-electromechanical systems (MEMS), nano-electromechanical systems (NEMS), and bio-MEMS, microscale and nanoscale materials exhibit mechanical properties and deformation mechanisms that are different from their bulk counterparts. Accurately predicting material response requires understanding fundamental mechanisms of material deformation and fracture occurrence in microscale and nanoscale. Material properties typically cannot be extrapolated from their respective bulk values, since material behavior often is not only different in microscale and nanoscale, but is also significantly affected by microstructure, sample size, and/or fabrication processes, and further is very sensitive to the influences of interfaces and adjoining materials. Changes in grain size and sample texture can lead to different responses even for the same materials. Some deformation mechanisms that are unimportant at bulk scale can become dominant as the sample's volume decreases and the relative surface area increases, such as in thin films. Samples made by gas deposition may exhibit different characteristics from those obtained by chemical clustering. Samples fabricated by mechanical attrition of metal powders may behave differently than those segmented from bulk materials.
Experiments to determine mechanical behavior and deformation mechanisms at macroscale have been largely successful. Yet, as the need for using smaller-scale materials has increased, such as with the development of microelectronics and micro-sensors, it has become increasingly important to assess the mechanisms of deformation and failure of materials at microscale and nanoscale. However, due to the limited number of available testing techniques for microscale and nanoscale samples, characterizing materials at these small scales has been a challenge, and much effort has been put into developing apparatus and methods for testing.
In general, the tension test is the most extensively developed and widely used test for material behavior, and it can be used to determine nearly all aspects of the mechanical behavior of a material. The basic principle of the tension test is quite simple, but numerous variables affect results. General sources of variation in mechanical-test results include factors such as shape of the specimen being tested, method of gripping the specimen, method of applying the force, speed of elongation, etc. Also, the extent of deformation in tension testing is limited by necking.
Compression tests are alternative approaches that overcome the necking limitation. Compression tests can provide useful information on plastic deformation and failure, but certain precautions must be taken to assure a valid test of material behavior. A buckling mode occurs when the length-to-diameter (L/D) ratio of the test specimen is large. In addition, even slightly eccentric loading on nonparallel compression plates will lead to shear distortion. Therefore, small L/D ratios are normally desired to avoid buckling and provide accurate measurements of the deformation behavior of materials in compression. Friction is another source of anomalous deformation in compression testing of ductile materials.
At macroscale, uniaxial tension and compression tests are accomplished by gripping opposite ends of a test item within the load frame of a test instrument, and producing tension in the specimen along a single axis while measuring the specimen's response. When properly conducted, such tests provide force-deformation relations that can quantify several important mechanical properties of a material such as 1) elastic deformation properties (Young's modulus and Poisson's ratio), 2) yield strength and ultimate tensile strength, 3) ductility properties, and 4) strain-hardening characteristics. Consideration of these material characteristics is important for reliable and optimized design.
In situ uniaxial tests, such as in scanning electron microscope (SEM) or transmission electron microscope (TEM) chambers, can potentially be used to allow direct observation of the deformation mechanism for quantitative and qualitative analysis. In the microscale or nanoscale, however, certain challenges arise when loading specimens. Examples include gripping of the specimen, aligning of the specimen in the direction of the force (to minimize likelihood of invalidation of the test caused by flexural stress on the specimen and resultant premature failure), and generating small forces (e.g., on the order of micro-Newtons) with high resolutions.
Some of these challenges can be addressed by using a substrate layer that is usually very compliant and with known material properties along with the actual specimen to be tested. However, introduction of the substrate complicates the experimental analysis because the microscale material properties of the substrate itself may not be known accurately, and because the interface with the substrate may influence the mechanical behavior of the specimen.
For example, a prior method of fabricating freestanding aluminum films includes evaporating metal film on a glass slide covered with a water-soluble layer, releasing the thin film from the glass slide by immersing it in water, and gluing the film to grips of a nano-tensilometer with epoxy. However, problems of mounting the specimen and premature specimen failure invalidate a significant number of tests using this method, and experimental results from the tests have shown significant variation in measured elastic modulus and ultimate tensile strength.
Another prior fabrication technique includes sputtering metal film on glass slides and releasing the films by peeling the films off from a substrate. A motor-driven micrometer is used to produce elongation in the films, and a load cell is used to read the stress. Laser spots diffracted from the gratings on the specimen surface determine the strain with 0.002% resolution.
Yet another known method provides a piezoactuated tensile testing apparatus using Ti—Cu—Ti multilayer films with a length of 700 μm, a width of 200 μm, and a total thickness of 1.2 μm. The films are patterned on wafers by lithography, and are then released from the substrate by wet etching of the substrate. Such a tensile testing apparatus has been known to provide force and displacement resolutions of about 200 μN and 20 nm, respectively. Still another testing method uses piezoelectric actuators for displacement with a load cell, a laser interferometer, and a strain gauge-optical encoder assembly to measure force and displacement.
A more recent material testing method is disclosed in U.S. Pat. No. 6,817,255, issued Nov. 16, 2004 (the '255 patent) to Haque and Saif, which is incorporated in its entirety herein by reference. The '255 patent discloses an apparatus and method for uniaxial tensile testing of a thin film material. This apparatus allows quantitative study of thin metal films down to very small thicknesses. The compact size and displacement-based measurement of example devices in the '255 patent allows one to conduct in-situ quantitative and qualitative tensile testing in environments such as a TEM and an SEM.
An example apparatus disclosed in the '255 patent includes a testing stage (e.g., a compact MEMS-based chip) that includes a co-fabricated thin film specimen to be tested, held by at least one force sensor beam at a first longitudinal end and by a support structure at a second longitudinal end. An example support structure includes a longitudinal beam connected to the second longitudinal end and aligned with the tensile axis of the specimen, and a plurality of lateral support beams. The support beams reduce flexing of the specimen resulting from misalignment of the pulling direction with the tensile axis of the specimen. Preferred embodiments of the chip include a pair of structural springs fabricated for maintaining structural integrity between the first and second ends of the chip and for addressing misalignment. Markers (e.g., displacement gauges) may be provided for measuring displacement of longitudinal ends of the specimen and deflection of the force sensor beam.
To test the thin film sample (specimen), the ends of the stage are separated (e.g., pulled) from one another by an actuator, such as a piezoactuator in SEM or a motor in TEM, which provides a tensile load on the sample. Measured displacement is used to determine material properties of the thin film specimen. For example, the force on the sample is determined from the displacement and the spring constant of the force sensor beams(s). The spring constant may be determined mathematically given dimensions and properties of the force sensor beams and/or by calibration, such as by using a nanoindenter. Sample stretching may be measured, for instance, by measuring displacement of the force sensor beams and the support structure.
Another testing stage for testing thin film samples is disclosed in Han, J. and Saif, M. T. A., “In Situ microtensile stage for electromechanical characterization of nanoscale freestanding films”, Review of Scientific Instruments, Vol. 77, No. 4, pp. 45102-1-8, 2006 (“Han and Saif”), which is incorporated in its entirety herein by reference. An example embodiment disclosed in Han and Saif uses a testing stage co-fabricated with a thin film specimen, as with the '255 patent. The specimen is disposed between a support structure with a longitudinal beam axially aligned with a tensile axis and a plurality of lateral beams at one end, and by one or more deformable, lateral force sensor beams with a bisecting longitudinal beam at the opposite end. To protect the metal thin film sample from possible premature failure during fabrication of the testing stage, a protecting beam is provided. The protecting beam extends parallel to the co-fabricated sample and connects the support structure to the longitudinal beam bisecting the force sensor beams.
After fabricating the sample, the protecting beam is cut using focused ion beam (FIB) to provide a displacement gauge. A laterally extending beam disposed between the support structure and the force sensor beams provides a reference displacement gauge. Tensile testing is performed similarly to that described in the '255 patent. The displacement gauges measure displacement of the sample and the force sensor beams, and they are sufficiently near the sample to allow simultaneous observation of the sample stress-strain and the displacement gauges in an observation chamber.
U.S. patent application Ser. No. 11/897,927 to Han et al., filed Aug. 31, 2007, incorporated in its entirety herein by reference, discloses methods and apparatus for testing a microscale or nanoscale sample using an assembly approach, which allows a sample to be fabricated independently of the testing stage. A testing stage comprises a frame having first and second laterally opposing ends, first and second side beams, and first and second longitudinal beams. Each of a pair of slots disposed at each of the free ends of the first and second longitudinal beams comprises a tapered portion leading to a generally longitudinal portion. The slots provide a seat for a dogbone-shaped sample.